Methods for operating a motor vehicle driven by an internal combustion engine and by two electrical machines

ABSTRACT

Two methods for operating a motor vehicle driven by an internal combustion engine (VM) and by two electrical machines (E 1 , E 2 ) are proposed. The motor vehicle has a transmission (G) with two power paths (LP 1 , LP 2 ) controllable independently of one another. Each power path (LP 1 , LP 2 ) is coupled via a respective epicyclic gear (P 1 , P 2 ) to one of the electrical machines (LP 1 , LP 2 ) and to the input shaft (KW) of the transmission (G) and can be coupled via shiftable gears ( 1, 2, 3, 4, 5, 6,  R) to the output shaft (AW) of the transmission (G). The object is to make it possible to operate the motor vehicle more efficiently. This is achieved in that to reduce circulating mechanical power flow between the two power paths (LP 1 , LP 2 ), particularly in the boost mode or upon recuperation of braking energy, the gear combination ( 1, 2, 3, 4, 5, 6,  R) is varied, or the engine rpm is lowered, or only one electrical machine (E 1 , E 2 ) is used as a generator. More-comfortable operation is additionally achieved in that upon electrical starting, the engine (VM) is started simultaneously.

[0001] The invention is based on a method as generically defined by thepreamble to the independent claims.

[0002] One such method is known for instance from German PatentDisclosure DE 199 03 936 A1. In this method, a motor vehicle is operatedat least intermittently by an internal combustion engine and at leastintermittently by at least one of two electrical machines. The motorvehicle has a transmission, with an input shaft, an output shaft, andtwo independently controllable power paths. Each power path is coupledto one of the two electrical machines and to the input shaft via arespective epicyclic gear and can be coupled to the output shaft viashiftable gears. If one gear is selected in each power path, then powercan be transmitted to the output shaft simultaneously over both powerpaths.

[0003] This method has the advantage that with a transmission ofrelatively simple mechanical construction, good efficiency can beachieved. However, the possibility exists that a circulating mechanicalpower flow can arise in the power paths. Moreover, upon startup orstarting of the internal combustion engine, one additional gear mustfirst be selected, which at that instant in the affected power pathcauses an interruption in the tractive force. Then the engine brake mustalso be released. Both situations involve losses, which reduce theefficiency.

[0004] The object of the invention is to refine the known method suchthat more-efficient operation of a motor vehicle is possible. Thisobject is attained by the characteristics of the independent claims.

[0005] The methods of the invention having the definitivecharacteristics of the bodies of the independent claims have theadvantage that compared to the known method, still more-efficientoperation is possible, because on the one hand a circulating mechanicalpower flow in the transmission is at least reduced; on the other, thereis no interruption of tractive force from selection of a gear. The powerof the engine can already be made available upon startup. Moreover,bucking of the motor vehicle upon selection of a gear is precluded fromoccurring. Furthermore, an engine brake can be dispensed with.

[0006] To detect the circulating mechanical power, the torques generatedby the engine and by the electrical machines can be ascertained andevaluated. It is also possible, as an alternative or in addition, tocompare the torques and their signs with one another. This detection canbe done with the data known from the control units.

[0007] In an especially simple possibility of electrical startup orstarting of the engine is, with gears selected in both power paths,simply to reverse the torque of one electrical machine, or to increasethe torque of one electrical machine and reduce the torque of the otherelectrical machine. Moreover, there is no interruption in tractive forceupon starting of the engine.

[0008] Starting up electrically with two forward gears that are farapart from one another and are disposed each in different power paths,there is the advantage that even at low vehicle speeds, the electricalmachines can be operated at a favorable rpm and with good efficiency.Moreover, a relatively wide gear ratio range is covered with this gearcombination, and at the same time engine starting from the electricaldriving mode is made possible. The gear ratio range covered isespecially wide if the startup is done with first gear and the highestgear, in particular sixth gear.

[0009] If electrical starting is done with reverse gear in one powerpath and second gear in the other power path, then vibration especiallyupon engine starting is well damped. This effect is improved stillfurther if the amount of the gear ratio of reverse gear is equal to theamount of the gear ratio of second gear.

[0010] If no generation of electrical energy is desired or possible, forinstance because the battery is full, then one electrical machine isoperated as a generator, which feeds the energy it generates into theother electrical machine which is operated as a motor.

[0011] Further advantages and advantageous refinements of the methods ofthe invention will become apparent from the dependent claims and thedescription.

[0012] One exemplary embodiment of the invention is shown in the drawingand described in further detail in the ensuing description. Shown are

[0013]FIG. 1, an elevation view of a transmission;

[0014]FIG. 2, the topology of the transmission of FIG. 1;

[0015]FIG. 3, the function of the basic elements of the transmission;

[0016]FIG. 4, the power flow in the transmission in stationaryoperation;

[0017]FIG. 5, the power flow upon electrical startup with gears R and 2at the instant of engine starting;

[0018]FIG. 6, the power flow upon electrical startup with gears R and 2during engine starting;

[0019]FIG. 7, the power flow upon electrical startup with gears 1 and 6just before engine starting;

[0020]FIG. 8, the power flow upon electrical startup with gears 1 and 6at the instant of engine starting;

[0021]FIG. 9, the power flow upon electrical startup with gears 1 and 6during engine starting;

[0022]FIG. 10, the power flow in stationary operation with gears 1 and6;

[0023]FIG. 11, the power flow upon startup without battery input;

[0024]FIG. 12, the power flow shortly after the electrical startupwithout battery input;

[0025]FIG. 13, the power flow upon recuperation of braking energy withthe dragging mode of the engine;

[0026]FIG. 14, the power flow upon recuperation of braking energy withthe dragging mode of the engine with circulating power flow;

[0027]FIG. 15, the power flow upon recuperation of braking energywithout the dragging mode of the engine;

[0028]FIG. 16, the power flow upon recuperation of braking energywithout a circulating power flow;

[0029]FIG. 17, the power flow in the boost mode with a circulating powerflow; and

[0030]FIG. 18, the power flow in the boost mode without a circulatingpower flow.

[0031] In FIG. 1, part of a drive train of a motor vehicle, inparticular a transmission G, is shown, of the kind already known from DE199 03 936 A1. Here, the planet wheels of two planetary gears P1 and P2are driven by a crankshaft KW or input shaft, driven by the internalcombustion engine VM (FIG. 2), via a two-mass flywheel ZMS and via twogear ratios i_(A1) and i_(A2) of a branching point VZ. Instead of theplanetary gears P1 and P2, other epicyclic gears can also be used, suchas friction wheel epicyclic gears. The planetary gears P1, P2 are moresuitable, however, because of their efficiency. The two-mass flywheelZMS brings about a reduction in the excitation of vibration in thetransmission G from the engine VM.

[0032] Two electrical machines E1 and E2 are connected to the sun wheelsof the corresponding planetary gears P1 and P2 via the gear ratiosi_(E1) and i_(E2). The electrical machines E1, E2, which are connectedboth to one another, for instance via an electrical intermediatecircuit, and to the battery of the motor vehicle, are equipped withpower electronics for four-quadrant operation.

[0033] The transmission G has two power paths LP1, LP2, controllableseparately via the electrical machines E1, E2. These power paths areconstructed as follows: The ring gears of the planetary gears P1, P2 areconnected to two reduction gears S and L, which by means of slidingsleeves SM actuated by transmission actuators GS, the transmissionactuators GS being driven by motors M, can be connected to gear wheelsof the gears R, 1, 3, 5 and 2, 4, 6, respectively. R stands for reversegear; gears 1-6 are the forward gears. The gear wheels of gears 1-6 andR mesh with corresponding counterpart gear wheels on the output shaftAW. In operation, one gear R, 1, 3 or 5 and 2, 4 or 6, respectively, ineach reduction gear S, L is selected. The distribution of the torque tothe two shafts S, L is done in a known way by triggering of theelectrical machines E1 and E2.

[0034] Naturally still other variant arrangements of the planetary gearsP1, P2 are conceivable as well. For example, coupling the power of theinternal combustion engine via the respective ring gears, or uncouplingthe power from the reduction gears via the respective ribs can be named.

[0035] The simplified view in FIG. 2 shows the topology of thetransmission G. The transmission G has two independently controllablepower paths LP1 and LP2; each power path LP1, LP2 is coupled via arespective epicyclic gear (planetary gear P1, P2) to one of theelectrical machines E1, E2 and to the input shaft (crankshaft KW) of thetransmission G and can be coupled via shiftable gears (gears R and 1-6)to the output shaft AW of the transmission G, and power can betransmitted simultaneously over both power paths LP1, LP2. The type ofpower transmission is affected by the engine VM and by the electricalmachines E1, E2.

[0036] The arrows shown in FIGS. 3a-3 c indicate the moment, the rpm,and the power, which is proportional to the product of the moment andrpm, refer to the individual transmission elements.

[0037] In FIG. 3a, which shows the branching point VZ from one to twoshafts, the power flow from one to two shafts can be seen. The ratios ofthe rotary speeds to one another remain the same, and the torques aredivided, so that the sum must necessarily be zero.

[0038] From FIG. 3b, the power flow via a planetary gear P1 or P2 can beseen. Two rotary speeds can be selected arbitrarily, and the thirdrotary speed is dependent on that; the rpm of the rib is a weightedaverage of the two other rotary speeds. All the torques are divided at afixed ratio or are in a fixed ratio to one another, which is dependenton the number of teeth.

[0039]FIG. 3c shows the power flow at a gear ratio of the gears 1-6 orR. The rotary speed and the torque are converted at a ratio that dependson the number of teeth; for reverse gear R, the direction of torque andmoment changes as well. The transmitted power, as a product of therotary speed and the moment, remains constant.

[0040]FIG. 4 shows the power flow in the transmission G in stationaryoperation. The power flows from the engine VM via the crankshaft KW, isdivided via the shafts S and L and via the gears 1-6 and R and thus viathe two power paths LP1, LP2, and is transmitted to the output shaft AW.A small portion flows via the electrical machines E1, E2; some of it isdiverted for the on-board electrical system and for electrical losses.If one electrical machine E1 or E2 is free of moment, the other supportsthe full moment, and the power flows over only one power path LP1 orLP2. This is true for both power paths LP1, LP2. All the gear ratiosbetween these two states can be represented in a continuously variableway.

[0041] In FIG. 5, the power flow is shown upon electrical startup, withthe gears R and 2 already selected, at the moment of engine starting.The torque of the electrical machine E1 of the gear shaft S in the powerpath LP1 in which reverse gear R has been selected is increased, whilethat of the other electrical machine E2 is decreased accordingly. Thusthe torque at the output shaft AW remains the same. Because of theunevenly divided torques, an additional moment on the crankshaft KW iscreated, which starts the engine VM.

[0042] In FIG. 6, the power flow is shown upon electrical startup withgears R and 2 during engine starting. At this moment, the engine isalready turning over, but ignition has not yet occurred. The electricalmachine E1 must produce high power then, in order simultaneously todrive the motor vehicle and to start the engine VM. As a rule, this canbe done successfully in warm starting and at low power takeoff levels.For example if 5 kW are needed upon starting of the engine VM, then theelectrical machine E1, at a maximum of 10 kW of transient power, canstill furnish 5 kW for the power takeoff. In addition, 1 kW, forexample, is furnished by the electrical machine E2, so that 6 kW at thepower takeoff are possible.

[0043] This method, in which the engine VM is started immediately afterthe electrical startup, has the advantage that even during the processof starting the engine, a power takeoff level is available at the wheeland can be used to accelerate the vehicle. This is used, particularly instop and go operation of the vehicle, to increase comfort, since as soonas the condition is tripped the driver can start up for starting theengine. The fact that reverse gear R disposed in one power path LP1 andsecond gear disposed in the other power path LP2 have been selected hasa favorable effect on vibration, because of the contrary rotation of thepower paths LP1 and LP2. This effect is improved further if the value ofthe gear ratio of reverse gear is equal to the value of the gear ratioof second gear, but the two gears R and 2 have opposite signs.

[0044] From FIG. 7, the power flow can be seen upon electrical startup,with first and sixth gears already selected, shortly before enginestarting. In electrical startup with first and sixth gears, theelectrical machine E1 acts as a motor. Its torque is boosted twice atthe planetary gear P1 and at first gear. The second electrical machineE2 withstands the reaction moment on the crankshaft KW. The result is areversed power flow in the power path LP2, and the second electricalmachine E2 acts as a generator. Because of the gear ratio of sixth gear,the rotary speeds and thus the power levels in this power path LP2remain low.

[0045] In FIG. 8, the power flow upon engine starting is shown. Here,the vehicle starts up electrically with first and sixth gears. Toinitiate the starting of the internal combustion engine VM, the momentequilibrium between the electrical machines E1 and E2 is cancelled. Themoment of the electrical machine E2 is increased markedly, while that ofthe electrical machine E1 is increased only slightly. Thus the powertakeoff moment remains constant; the power flow in the transmission Gincreases; and at the crankshaft KW, a torque is present that leads toengine starting.

[0046] In FIG. 9, the power flow can be seen shortly after enginestarting. The engine VM already has a rotary speed but ignition has notyet occurred. The rotary speeds and thus the power flow now changeconsiderably. The rotary speed and motor output of the electricalmachine E1 decrease slightly; the electrical machine E2 changes itsdirection of rotation and changes from operation as a generator tooperation as a motor. Power circulates in the transmission G. The powerflow is supplied from both electrical machines E1, E2. The engine VM andthe output shaft AW are sinks for the power.

[0047] If—depending on the driver demand and vehicle and/orenvironmental status—starting is either done electrically and/or theengine VM is started, it is accordingly important that with gearsselected in both power paths, the torque of at least one electricalmachine E1, E2 is varied. Concretely, this can be done such that thetorque or direction of the torque of one electrical machine E1, E2 isreversed, or that the torque of one electrical machine E1, E2 isincreased while the torque of the other electrical machine E1, E2 isreduced. Under certain circumstances, a variably pronounced increase inboth torques is also conceivable. These methods have the advantage ofbeing simple to control, since only the electrical machines E1, E2 haveto be triggered accordingly. Moreover, an interruption in tractive forcein one of the two power paths LP1, LP2 is avoided. This results not onlyin increased efficiency but also in increased comfort, since uponstartup the motor vehicle has less tendency to bucking.

[0048] In FIG. 10, the power flow in stationary operation is shown withfirst and sixth gears. After the end of the starting process, ignitionof the engine VM has occurred, and the engine produces torque and power.The torques and the power flow change. Immediately and without furthershifting, a typical operating state with the paired gears 1 and 6ensues. The pairing of first and sixth gears is highly suitable for lowvehicle speeds, among other purposes. The electrical machines E1, E2then have favorable operating rotary speeds. By controlling the momentflow, it is possible to adjust all the gear ratios between first andsixth gears in a continuously variable manner. Depending on what isrequired, for instance if the vehicle speed or load is increasing, thisgear combination can be replaced by first and second gears, first andfourth gears, third and sixth gears, or fifth and sixth gears, forinstance.

[0049] What is critical here is accordingly that the startup is doneelectrically, with two forward gears 1 and 6, disposed in the powerpaths LP1, LP2 and located far apart and each in different power pathsLP1, LP2. As a result, a relatively wide vehicle speed range can becovered, and a relatively high starting moment for the engine can beachieved. This becomes especially favorable if starting up is done withfirst gear and the highest gear, in particular as in this case withsixth gear.

[0050] In FIG. 11, the power flow upon startup with the aid of theengine VM without battery input is shown. This is required for instanceif the battery is fully charged and no further load is to be put on it.Then one electrical machine E2 is operated as a generator, and oneelectrical machine E1 is operated as a motor. For that purpose, thegears R and 2 are for instance selected. With the engine VM running andthe electrical machines E1, E2 rotating, two differently orientedtorques are now established at the electrical machines E1, E2. Theelectrical machine E1 in the power path LP1 with reverse gear R acts asa motor. The electrical machine E2 in the power path LP2 that has secondgear acts as a generator. The resultant electrical power is suppliedagain (minus losses or on-board electrical system requirements) to theelectrical machine E1 acting as a motor, so that the battery is notinvolved. Because of the reversal of the direction at reverse gear R,the torques at the output shaft AW add up again. As long as the motorvehicle is stopped, the engine VM need merely compensate for the powerloss and therefore produces only slight moments.

[0051] In FIG. 12, the power flow is shown shortly before startupwithout battery input. If the motor vehicle begins to move, power flowsto the output shaft AW. If the engine rpm remains constant, the rotaryspeeds of the two electrical machines E1, E2 increase and decrease, andthus so do the moments that can be transmitted for the same electricalpower. The power of the engine VM then increases, and the additionalpower flows to the output shaft AW. As operation continues, the powerpath LP1 becomes completely moment-free; reverse gear R is then shiftedout of and replaced with a forward gear (typically third gear, but firstor fifth gear is also possible). The mode of operation changes over tothe standard situation for stationary operation.

[0052] An advantage of the gear combination R and 2 is in particular thepossibility upon startup of avoiding or at least diminishing both anincreased burden on the on-board electrical system and a circulatingmechanical power in the transmission G.

[0053] In FIG. 13, the power flow is shown upon recuperation of brakingenergy in the dragging mode of the engine VM. For the recuperation, inthe simplest case, the torques at the engine VM and at the output shaftAW are reversed. The power takeoff is then braked and the engine VM isdragged. As a result, the electrical machines E1, E2 also exchange rolesas a motor and generator. To avoid or diminish a circulating mechanicalpower, only the electrical machine E2 acting as a generator istriggered. The other electrical machine E1 runs with it, without a load.

[0054] In this operating state, the power flows from the output shaft AWto the engine VM and to the electrical machine E2. That is, the engineVM can be put into the overrunning mode, in which no fuel is consumed.In this operating state, lesser recuperation power levels with highefficiency can be achieved. Shifting is unnecessary to change over fromthe normal driving mode into this operating mode. For avoiding ordiminishing circulating mechanical power between the two power paths,particularly upon the recuperation of braking energy, it is advantageousto use only one electrical machine as a generator.

[0055] In FIG. 14, in contrast to FIG. 13, a circulating mechanicalpower between the two power paths LP1, LP2 upon recuperation of brakingenergy in the dragging mode of the engine VM is shown. This occurs atrelatively high recuperation power levels. In this case, the otherelectrical machine E1 is added as a generator. At the power takeoff, ahigher braking moment can be generated, and more total current can begenerated at two electrical machines E1, E2. This situation can also bereached directly from the driving mode, without an additional shiftingoperation. However, the result is the circulating mechanical power,which means poorer mechanical efficiency of the transmission.

[0056] In FIG. 15, the power flow is shown upon recuperation of brakingenergy without a dragging mode of the engine. By means of a suitableregulation of the electrical machines E1, E2, it is also possible inthis operating state to completely circumvent the dragging mode of theengine VM. In that case, the torques that act from the two power pathsLP1, LP2 on the crankshaft KW cancel one another out. In this operatingstate, the idling governor of the engine VM must keep the engine at itsrotary speed. To that end, the idling consumption at the applicable rpmmust be brought to bear. The operating state is therefore especiallyappropriate if the braking power at the transmission output that thedriver is demanding is inadequate to produce the drag power of theengine VM at its lowest possible operating rpm.

[0057] In FIG. 16, the power flow is shown upon recuperation of brakingenergy without circulating power flow. To circumvent the circulatingpower flow in the transmission G, different rpm conditions can beestablished by means of shifting. The power flow can thus be directeddifferently. This is preferably performed whenever relatively longrecuperation phases are expected, such as when driving some distanceuphill or downhill. In that situation, the shifting has especially highusefulness because of the improvement in the mechanical efficiency ofthe transmission. At the same time, the shifting preselects a gearcombination for normal driving operation at a slower speed, whichtypically occurs after the recuperation operation.

[0058] Analogously to the recuperation, in FIG. 17 a boost mode can alsobe realized. Here both the engine VM and both electrical machines E1, E2are used for driving. However, the consequence is a circulatingmechanical power, which leads to poorer transmission efficiency. On theother hand, a high additional power into the drive train is achieved.

[0059] To avoid the circulating power flow, in FIG. 18 a different gearcombination can be selected in the boost mode as well. The requisiteshifting operation is equivalent to downshifting, which is alsonecessary for conventional transmissions when there is a high powerdemand. Alternatively, lowering the rpm of the engine VM while thevehicle speed remains constant is also possible. This is equivalent to achange in the total gear ratio of the engine/transmission output, forgear stages that remain the same. However, the higher power demand inthe boost mode is in contradiction to this.

[0060] To diminish or avoid circulating power flow between the two powerpaths LP1, LP2, particularly in the boost mode or in the recuperation ofbraking energy, the gear ratio combination or the engine rpm can bevaried, and in particular lowered, or only one electrical machine E1, E2can be used as a generator or motor.

[0061] For detecting the circulating power, the torques generated by theengine VM and the electrical machines E1, E2 can be evaluated along withtheir rotary speeds. The torques can be ascertained from the existingcontrol unit or units of the engine VM and of the electrical machinesE1, E2. By comparing the torque with set-point values, which areavailable for instance in tables, it can be determined whethermechanical power is being lost from circulation between the two powerpaths LP1, LP2. Alternatively or in addition, the existence of acirculating mechanical power can advantageously also be checked byevaluating the sign of the torques, since the signs are also anindicator of circulating mechanical power. The magnitude of thecirculating power flow can be calculated by taking into account therotary speeds of the engine VM and electrical machines E1, E2. Ashifting operation, a change in particular reduction in the engine rpm,or the use of only one electrical machine E1, E2 as a generator can bemade dependent on this.

1. A method for operating a motor vehicle that is driven at leastintermittently by an internal combustion engine (VM) and at leastintermittently by at least one of two electrical machines (E1, E2),which has a transmission (G) with two power paths (LP1, LP2) that arecontrollable independently of one another, each power path (LP1, LP2)being coupled via a respective epicyclic gear (P1, P2) to one of theelectrical machines (LP1, LP2) and to the input shaft (KW) of thetransmission (G) and being couplable to the output shaft (AW) of thetransmission (G) via shiftable gears (1, 2, 3, 4, 5, 6, R), power beingtransmissible simultaneously over both power paths (LP1, LP2),characterized in that at least to reduce circulating mechanical powerflow between the two power paths (LP1, LP2), particularly in the boostmode or upon recuperation of braking energy, the gear combination (1, 2,3, 4, 5, 6, R) or the engine rpm is varied, or only one electricalmachine (E1, E2) is used as a generator.
 2. The method of claim 1,characterized in that to detect the circulating mechanical power, thetorques generated by the engine (VM) and by the electrical machines (E1,E2) are ascertained and compared with predetermined set-point values. 3.The method of claim 1 or 2, characterized in that to detect thecirculating mechanical power, the sign of the torques generated by theengine (VM) and the electrical machines (E1, E2) are compared with oneanother.
 4. A method for operating a motor vehicle that is driven atleast intermittently by an internal combustion engine (VM) and at leastintermittently by at least one of two electrical machines (E1, E2),which has a transmission (G) with two power paths (LP1, LP2) that arecontrollable independently of one another, each power path (LP1, LP2)being coupled via a respective epicyclic gear (P1, P2) to one of theelectrical machines (LP1, LP2) and to the input shaft (KW) of thetransmission (G) and being couplable to the output shaft (AW) of thetransmission (G) via shiftable gears (1, 2, 3, 4, 5, 6, R), power beingtransmissible simultaneously over both power paths (LP1, LP2),characterized in that for electrical startup and/or for starting theengine (VM) with gears (1, 2, 3, 4, 5, 6, R) selected in both powerpaths, the torque of at least one electrical machine (E1, E2) is variedas a function of the driver demand, vehicle and/or environmental status.5. The method of claim 4, characterized in that the torque of oneelectrical machine (E1, E2) is reversed.
 6. The method of claim 4,characterized in that the torque of the one electrical machine (E1, E2)is increased, and the torque of the other electrical machine (E1, E2) isreduced.
 7. The method of one of claims 4-6, characterized in thatimmediately after the electrical startup, the engine (VM) is started. 8.The method of one of claims 4-7, characterized in that with two forwardgears (1, 6) disposed on the power paths (LP1, LP2), which forward gearsare as far as apart as possible and are each disposed in different powerpaths (LP1, LP2), electrical starting is done or the engine (VM) isstarted.
 9. The method of one of claims 4-8, characterized in that uponstartup or starting of the engine (VM), reverse gear (R) disposed in onepower path (LP1) and the lowest gear (2) disposed in the other powerpath (LP2) are selected.
 10. The method of one of claims 4 or 9,characterized in that upon startup after starting of the engine (VM) theelectrical machine (E1) in the power path (LP1) of reverse gear (R) isoperated as a generator, and the electrical machine (E1) in the otherpower path (L2) is operated as a motor.
 11. A transmission forperforming the method of claim 1 or 4, characterized in that oneelectrical machine (E1, E2) is operated as a generator, and the otherelectrical machine (E1, E2) is operated as a motor.
 12. The transmissionfor performing the method of claim 1 or 4, characterized in that theamount of the gear ratio of reverse gear (R) is equal to the amount ofthe step-up of second gear (2), and the two gears (R, 2) have oppositesigns.